There has been a strong demand for reduction of the amount of carbon dioxide in order to deal with atmospheric pollution and global warming. In the automobile industry, the reduction of emissions of carbon dioxide is highly expected in association with the spread of electric vehicles (EV) and hybrid electric vehicles (HEV). Thus, development of electric devices such as secondary batteries for driving motors as a key to practical application of such vehicles, is actively being carried out.
The secondary batteries for driving motors are required to have quite high output performance and high energy as compared with lithium ion secondary batteries for general use in mobile phones, laptop computers and the like. Therefore, lithium ion secondary batteries having the highest theoretical energy among all types of batteries are gaining increasing attention, which is leading to rapid development of the lithium ion secondary batteries.
A lithium ion secondary battery generally includes: a positive electrode including a positive electrode current collector to which a positive electrode active material and the like is applied on both surfaces via a binder, a negative electrode including a negative electrode current collector to which a negative electrode active material and the like is applied on both surfaces via a binder, and an electrolyte layer, the positive electrode and the negative electrode being connected to each other via the electrolyte layer and housed in a battery case.
In such a conventional lithium ion secondary battery, a carbon/graphite-based material having the advantage of charge-discharge cycle life or costs has been used for the negative electrode. However, the carbon/graphite-based negative electrode material has the disadvantage that a sufficient theoretical charge-discharge capacity of 372 mAh/g or higher obtained from LiC6 as a lithium introduction compound accounting for the largest amount, cannot be ensured because the battery is charged/discharged by absorbing lithium ions into graphite crystals and releasing the lithium ions therefrom. As a result, it is difficult to ensure a capacity and energy density sufficient to satisfy vehicle usage on the practical level by use of the carbon/graphite-based negative electrode material.
On the other hand, a battery using a material alloyed with Li for a negative electrode has higher energy density than the conventional battery using the carbon/graphite-based negative electrode material. Therefore, such a negative electrode material is highly expected to be used for a battery in a vehicle. For example, 1 mole of a Si material absorbs and releases 4.4 moles of lithium ions, in accordance with the following reaction formula (A), during charge and discharge, and a theoretical capacity of Li22Si5 (═Li4.4Si) is 2100 mAh/g. Further, the Si material has an initial capacity as high as 3200 mAh/g in the case of calculation per Si weight.[Chem. 1]Si+4.4Li++e−Li4.4Si  A
However, in the lithium ion secondary battery using the material alloyed with Li for the negative electrode, expansion-shrinkage in the negative electrode at the time of charge and discharge is large. For example, volumetric expansion of the graphite material in the case of absorbing Li ions is approximately 1.2 times. However, the Si material has a problem of a reduction in cycle life of the electrode due to a large volumetric change (approximately 4 times) which is caused by transition from an amorphous state to a crystal state when Si is alloyed with Li. In addition, when using the Si negative electrode active material, a capacity has a trade-off relationship with cycle durability. Thus, it is difficult to increase the capacity and improve the cycle durability concurrently.
In order to deal with the problems described above, there is known a negative electrode active material for a lithium ion secondary battery containing an amorphous alloy having a formula: SixMyAlz (for example, refer to Patent Document 1). In the formula, x, y, and z represent atomic percent values and satisfy the conditions of x+y+z=100, x≥55, y<22, and z>0, and M is a metal formed of at least one of Mn, Mo, Nb, W, Ta, Fe, Cu, Ti, V, Cr, Ni, Co, Zr, and Y. Patent Document 1 teaches in paragraph [0018] that good cycle life is ensured in addition to a high capacity by minimizing the content of the metal M.